This invention relates to a twin wire former for application to the wire part of a paper machine.
An example of a conventional twin wire former is shown in FIG. 2: dewaterers 5, 7 are inside the loop of a wire 4, and a dewatering zone is configured so that the wire-run constituting the forming zone thereof is substantially vertical. A couch roll 8a is a suction roll, and the wet paper is transferred to the wire 4 on the couch roll 8a. In FIG. 1, reference numeral 1 denotes a head box, 2 is a raw material jet, 3 is a wire, 10 is a breast roll and 11 is a forming roll.
In another example of a conventional twin wire former shown in FIG. 3, dewaterers 6 and 7 are disposed alternately inside wire loops 3 and 4b; the first dewaterer is a roll 10a with a small radius of curvature, and as in FIG. 2 the couch roll 8b is a suction roll and the wet paper is transferred to the wire 4 as it passes over the couch roll 8b.
FIG. 4 shows a further example of a conventional twin wire former. The twin wire former in this case consists of diagonal twin wire loops; dewaterers 5c, 6c, 7c are disposed alternately inside the loops of two wires 3c, 4c, and the wet paper is transferred to the wire 4c on a curved transfer box. The dewaterer 5c has a small radius of curvature, and the couch roll 8c is a suction roll. The lower wire 4c passes around a breast roll 10c, the dewaterer 5c, a separating suction unit 9c, the dewaterer 7c and the couch roll 8c. The upper wire 3c passes around a forming roll 11c, the dewaterer 6c, and a tension roll 13c.
In the twin wire former shown in FIG. 2, because the dewaterers 5, 7 are both disposed inside the loop of the same wire 4, the effect of the dewaterers 5, 7 on the formation of the paper layer is asymmetrical. Also, because the couch roll 8a is a suction roll, the initial cost is high and because a large vacuum airflow is required the energy costs are also high. Furthermore, there are problems such as that because the top wire return roll 14a is swung up to replace the wires 3 and 4, the height required for the machine is large.
In the twin wire former shown in FIG. 3, because the initial dewatering is performed by the roll 10a having a small radius of curvature, this dewatering is sudden. Also, as in the case shown in FIG. 2, because the top wire return roll 14a is swung up for replacement of the wires, the height of the machine is large. Furthermore, because there are two suction couch rolls, there are the same problems of the initial cost and the energy costs being high as in the case shown in FIG. 2.
In the twin wire former shown in FIG. 4, the initial dewatering is sudden dewatering with a roll, as in FIG. 3, and as in the cases shown in FIG. 2 and FIG. 3 there are problems associated with the use of the suction couch roll 8.